3N +3 switch module cascade-based back-to-back multi-port conversion circuit and capacity expansion method

Abstract

A back-to-back multi-port conversion circuit based on 3N + 3 switch module cascading comprises m input end hexagonal star type cascade circuits, six multi-port converters and n output end hexagonal star type cascade circuits, one end of each input end hexagonal star type cascade circuit is connected with a generator through a three-phase line, and the other end of each input end hexagonal star type cascade circuit is connected with a power supply through a three-phase line. The other end of the input end hexagonal star type cascade circuit is connected with alternating current input ports of the six multi-port converters through six groups of three-phase lines respectively; one end of the output end hexagonal star-shaped cascade circuit is connected with alternating current output ports of the six multi-port converters through six groups of three-phase lines, and the other end of the output end hexagonal star-shaped cascade circuit 3 is output through the three-phase lines. According to the design, the frequency is completely decoupled, the port voltage and current waveform effects are good, the topological port expansion is very flexible, and the developable potential functions are very powerful.

Description

Technical Field

[0001] The present invention relates to a back-to-back multi-port conversion circuit and an expansion method based on cascading of 3N+3 switch modules, and is specifically applicable to the field of offshore wind power. Background Art

[0002] The clean and intelligent development of energy has become an irresistible trend. Due to advantages such as clean environmental protection, rich resources, stable wind speed, less land occupation for development, and small negative environmental impacts, offshore wind power has great development prospects. Currently, the installed capacity of offshore wind power in China's coastal areas has been growing rapidly. However, with the increasing saturation of coastal resources, deep water and far sea areas, and large megawatt units have become the future direction of offshore wind power. Some studies have shown that the fractional frequency transmission (FFTS) technology has great prospects for the application of medium and long-distance offshore wind power transmission.

[0003] In recent years, the installed capacity of offshore wind power has been growing rapidly. In 2018, the newly installed capacity of offshore wind power reached 1.65 million kilowatts, with a year-on-year increase of 42.7%. With the gradual saturation of coastal resources, deep water and far sea areas, and large megawatt units have become the future direction of offshore wind power. How to economically and efficiently achieve the long-distance transmission and aggregation of large-capacity offshore wind power has become a current research hotspot.

[0004] The existing technical solutions for offshore wind power transmission mainly include three types: high voltage alternating current (HVAC), high voltage direct current (HVDC), and fractional frequency transmission system (FFTS). HVAC uses the traditional power frequency transmission method, with a simple structure and rich operation experience; however, for long-distance and large-capacity wind power transmission, due to the sharp increase in the capacitive charging current of the cable, the line loss increases, and the effective utilization rate of the line capacity is greatly reduced, so it is particularly suitable for short-distance and small-capacity wind power transmission. HVDC is affected by the construction investment and operation and maintenance of the offshore converter platform and is mainly suitable for long-distance wind power transmission. The fractional frequency transmission system FFTS refers to reducing the electrical distance of the AC transmission line, increasing the transmission capacity, reducing the number of transmission circuits and the outgoing line corridor by low-frequency power transmission (such as 50 / 3 Hz) without increasing the voltage level. For medium and long-distance large-capacity wind power transmission, FFTS has better technical and economic advantages.

[0005] As the core device for frequency - divided power transmission, the cycloconverter needs to achieve the integration of low - frequency wind power systems and power - frequency power grids. In existing cycloconverters, due to the direct coupling between ports, multiple - frequency power components appear in each bridge arm. Coupled with the double - frequency fluctuation of single - phase instantaneous power, the DC - side capacitor voltage fluctuation of its units is complex, which in turn affects the waveform quality of port power and control difficulty. In addition, due to different control commands for each bridge arm, a large circulating - current component appears in the topological loop structure, exacerbating the device current stress and power loss. Moreover, when expanding the ports of this type of topology, the number of required devices will increase exponentially. As the scale of the wind farm gradually expands and there are multiple groups of wind - power integration requirements, the volume and cost of such converter equipment will increase significantly. Summary of the Invention

[0006] The object of the present invention is to overcome the problems existing in the prior art and provide a back - to - back multi - port conversion circuit and an expansion method based on cascading 3N + 3 switch modules. The 3N + 3 switch converter derived from the nine - switch converter is very suitable for the expansion of multiple ports: for each expanded port, capacity expansion is carried out within the module, and the unit output ports in the topology are all three - phase. When symmetrical, the instantaneous power is a constant, and the DC - side capacitor voltage has no ripple fluctuation. When applied to the interconnection of different - frequency systems, due to the isolation effect of the DC - side capacitor, complete frequency decoupling can be achieved, so the waveforms of port voltage and current are good. In addition, because offshore wind power has a constant - frequency characteristic, the disadvantage of low DC - voltage utilization rate of the nine - switch converter is effectively avoided. Therefore, a large - capacity multi - port converter topology based on cascading 3N + 3 switch modules is studied to enable the power of m groups of wind - power system ports to be integrated into n groups of power - grid lines for grid connection. This converter has higher integration, economy, and more excellent control performance, which will also be beneficial to the power - balance stability among multiple groups of wind - power systems and the comprehensive regulation of grid - connection performance. The relevant topology has been proven to operate efficiently and reliably in the drive applications of two motors.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] In the first aspect, the present invention provides a back - to - back multi - port conversion circuit based on cascading 3N + 3 switch modules, including: m input - end hexagonal - star cascading circuits, six multi - port converters, and n output - end hexagonal - star cascading circuits. One end of the input - end hexagonal - star cascading circuit is connected to a generator through three - phase lines, and the other end of the input - end hexagonal - star cascading circuit is respectively connected to the AC input ports of the six multi - port converters through six groups of three - phase lines; one end of the output - end hexagonal - star cascading circuit is respectively connected to the AC output ports of the six multi - port converters through six groups of three - phase lines, and the other end of the output - end hexagonal - star cascading circuit outputs through three - phase lines.

[0009] The multi-port converter includes a three-phase rectifier bridge arm at the input end, a three-phase inverter bridge arm at the output end, and a transformer T. The three-phase rectifier bridge arm is shunted with a capacitor C1, and the three-phase inverter bridge arm is shunted with a capacitor C2. The high-voltage side of the three-phase rectifier bridge arm is connected to the high-voltage side of the three-phase inverter bridge arm 22 after being serially connected with the primary winding of the transformer T, and the low-voltage side of the three-phase rectifier bridge arm is connected to the low-voltage side of the three-phase inverter bridge arm after being serially connected with the secondary winding of the transformer T.

[0010] Each rectifier bridge arm in the three-phase rectifier bridge arm includes: m + 1 switch units connected in series in sequence, and a wiring port is arranged between adjacent switch units; the combination of the wiring ports of the three-phase rectifier bridge arm constitutes m three-phase AC input ports.

[0011] Each inverter bridge arm in the three-phase inverter bridge arm includes: n + 1 switch units connected in series in sequence, and a wiring port is arranged between adjacent switch units; the combination of the wiring ports of the three-phase inverter bridge arm constitutes n three-phase AC output ports.

[0012] In the input hexagonal star cascade circuit, one line is selected from each group of lines as the input to form an AC input with the same frequency and a 60-degree phase difference between adjacent signals, and the remaining lines are serially connected through inductors in sequence to form a hexagonal connection to connect six multi-port converters.

[0013] The AC input of the input hexagonal star cascade circuit is connected to a doubly-fed wind generator.

[0014] The AC input of the input hexagonal star cascade circuit forms two groups of three-phase lines and is respectively connected to two wind turbines.

[0015] In the output hexagonal star cascade circuit, one line is selected from each group of lines as the input to form an AC output with the same frequency and a 60-degree phase difference between adjacent signals, and the remaining lines are serially connected through inductors in sequence to form a hexagonal connection to connect six multi-port converters;

[0016] The AC output of the output hexagonal star cascade circuit forms two groups of three-phase lines and is connected to the power grid or electrical equipment.

[0017] In a second aspect, the present invention provides a method for expanding the capacity of a back-to-back multi-port conversion circuit based on 3N + 3 switch module cascading. The capacity expansion method is based on the back-to-back multi-port conversion circuit of 3N + 3 switch module cascading and includes the following steps:

[0018] S1 Calculate the increment m' of m in the circuit according to the number x of fans installed for capacity expansion. According to the original configuration data of the circuit and the configuration data of the newly added fans, calculate whether the maximum value of the relevant index parameters of the equipment operation meets the operation safety index of the existing three-phase inverter bridge arm.

[0019] If the safety index is met, only the input hexagonal star cascade circuit and the three-phase rectifier bridge arm 21 on the input side are expanded to obtain a new circuit design diagram;

[0020] If the safety index is not met, the three-phase inverter bridge arm is expanded, the minimum value of n that meets the safety index is calculated, and then the input and output hexagonal star cascade circuits, the three-phase rectifier bridge arm, and the three-phase inverter bridge arm are expanded simultaneously to obtain a new circuit design diagram;

[0021] S2 Install the newly added components and the input and output hexagonal star cascade circuits according to the circuit design.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. In the back-to-back multi-port conversion circuit based on 3N+3 switch module cascade of the present invention, this topology can realize the unified collection of multiple groups of low-frequency wind power systems and grid connection to the power frequency grid, which will help to balance the power between different wind farms and is more conducive to the centralized control of multiple ports. The power frequency port and the low-frequency port are frequency decoupled, and the port waveform effect is good. Since the intermediate DC link is omitted, when the power frequency grid is connected to the low-frequency wind power collection, there will inevitably be power coupling of different frequencies on the bridge arm, and this coupling power will be directly reflected on the DC side capacitor, causing its voltage to fluctuate at multiple frequencies, thereby exacerbating the waveform distortion of the port voltage and current. In this topology, the power frequency grid and the low-frequency system are isolated on the DC side of each unit, the frequencies are completely decoupled, and the port voltage and current waveform effects are good. The regenerative energy between multi-port loads can be directly utilized, improving the energy utilization efficiency and reducing the pressure on the feedback grid; at the same time, the equipment volume can also be reduced.

[0024] 2. In the back-to-back multi-port conversion circuit based on 3N+3 switch module cascade of the present invention, this topology has multiple power frequency terminals, so the expansion of the ports is diversified. It can not only realize the interconnection of multiple groups of wind power, but also provide ports for the connection of multi-terminal power grids, loads, the access of various compensation and regulation devices, and the voltage level can also be adjusted by the number of cascade units. Even more, a DC port system can be further led out from the DC side capacitor terminal of its unit. Therefore, the port expansion of this topology is very flexible, and the potential functions that can be developed are very powerful.

[0025] 3. In the rectifier side at the front end of each unit in a back-to-back multi-port conversion circuit based on 3N+3 switch module cascading according to the present invention, a hexagonal star cascading idea is adopted. After six inductors are connected in sequence to form a hexagon, it is interconnected with the power grid to achieve bidirectional energy flow. The inverter side also adopts the hexagonal star connection idea and is connected to the low-voltage system. The entire converter forms a completely symmetric 3m+3 rectification matching 3n+3 inversion back-to-back unit structure. The DC-side capacitor voltage is stable and easy to control, and the capacitor capacity is small. When the output ports of each unit in this topology are all three-phase symmetric, the instantaneous power is always constant. Therefore, there is no ripple fluctuation in the DC-side voltage. Coupled with the active front end, the voltage is stable and easy to control, and a smaller-capacity capacitor can meet the requirements.

[0026] 4. In the capacity expansion method of a back-to-back multi-port conversion circuit based on 3N+3 switch module cascading according to the present invention, it is possible to directly expand the capacity in the multi-port converter for the added fans, and at the same time ensure the safe and stable operation of the inverter circuit at the output end for capacity expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the circuit structure diagram of the present invention.

[0028] Figure 2 is Figure 1 the topology structure diagram of the multi-port converter in

[0029] Figure 3 is Figure 1 the topology structure diagram of the hexagonal star cascading circuit at the output end in

[0030] Figure 4 is Figure 3 the schematic diagram of the wiring method of

[0031] Figure 5 is the schematic diagram of the wiring method of Embodiment 2.

[0032] Figure 6 is the circuit structure diagram of Embodiment 2.

[0033] In the figure: the hexagonal star cascading circuit 1 at the input end, the multi-port converter 2, the three-phase rectifier bridge arm 21, the three-phase inverter bridge arm 22, and the hexagonal star cascading circuit 3 at the output end. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0035] Embodiment 1:

[0036] Refer to Figures 1 to 3, a back-to-back multi-port conversion circuit based on the cascading of 3N+3 switch modules, characterized by comprising: m input terminal hexagonal cascading circuits 1, six multi-port converters 2, and n output terminal hexagonal cascading circuits 3. One end of the input terminal hexagonal cascading circuit 1 is connected to a generator through a three-phase line, and the other end of the input terminal hexagonal cascading circuit 1 is respectively connected to the AC input ports of the six multi-port converters 2 through six groups of three-phase lines; One end of the output terminal hexagonal cascading circuit 3 is respectively connected to the AC output ports of the six multi-port converters 2 through six groups of three-phase lines, and the other end of the output terminal hexagonal cascading circuit 3 outputs through a three-phase line.

[0037] The multi-port converter 2 includes a three-phase rectifier bridge arm 21 at the input end, a three-phase inverter bridge arm 22 at the output end, and a transformer T. The three-phase rectifier bridge arm 21 is shunted with a capacitor C1, the three-phase inverter bridge arm 22 is shunted with a capacitor C2. The high-voltage side of the three-phase rectifier bridge arm 21 is connected to the high-voltage side of the three-phase inverter bridge arm 22 after being serially connected with the primary winding of the transformer T, and the low-voltage side of the three-phase rectifier bridge arm 21 is connected to the low-voltage side of the three-phase inverter bridge arm 22 after being serially connected with the secondary winding of the transformer T.

[0038] Each phase of the rectifier bridge arm in the three-phase rectifier bridge arm 21 includes: m+1 switch units connected in series in sequence, and a wiring port is arranged between adjacent switch units; The combination of the wiring ports of the three-phase rectifier bridge arms forms m three-phase AC input ports;

[0039] Each phase of the inverter bridge arm in the three-phase inverter bridge arm 22 includes: n+1 switch units connected in series in sequence, and a wiring port is arranged between adjacent switch units; The combination of the wiring ports of the three-phase inverter bridge arms forms n three-phase AC output ports.

[0040] The switch unit is an IGBT in parallel with a freewheeling diode, and U / V / W respectively correspond to the three-phase bridge arms.

[0041] In the input terminal hexagonal cascading circuit 1, one line is selected from each group of lines as the input to form an AC input with the same frequency and adjacent signals having a phase difference of 60 degrees, and the remaining lines are sequentially connected in series through inductors to form a hexagonal connection to connect the six multi-port converters 2.

[0042] The AC input of the input terminal hexagonal cascading circuit 1 is connected to a wind power doubly-fed generator.

[0043] In the output terminal hexagonal cascading circuit 3, one line is selected from each group of lines as the input to form an AC output with the same frequency and adjacent signals having a phase difference of 60 degrees, and the remaining lines are sequentially connected in series through inductors to form a hexagonal connection to connect the six multi-port converters 2;

[0044] The AC output of the output hexagonal star cascaded circuit 3 forms two groups of three-phase lines to connect to the power grid or electrical equipment.

[0045] A method for expanding the capacity of a back-to-back multi-port conversion circuit based on cascading 3N+3 switch modules. The capacity expansion method is based on the back-to-back multi-port conversion circuit cascading 3N+3 switch modules described in any one of claims 1 to 7, and includes the following steps:

[0046] S1 According to the number of fans x installed for capacity expansion, calculate the increment m' of m in the circuit. According to the original configuration data of the circuit and the configuration data of the newly added fans, calculate whether the maximum value of the relevant index parameters of the equipment operation meets the operation safety index of the existing three-phase inverter bridge arm 22;

[0047] If the safety index is met, only expand the capacity of the input hexagonal star cascaded circuit and the three-phase rectifier bridge arm 21 on the input side to obtain a new circuit design diagram;

[0048] If the safety index is not met, expand the capacity of the three-phase inverter bridge arm 22, calculate the minimum value of n that newly meets the safety index, and then expand the capacity of the input and output hexagonal star cascaded circuits and the three-phase rectifier bridge arm 21 and the three-phase inverter bridge arm 22 at the same time to obtain a new circuit design diagram;

[0049] S2 According to the circuit design, install the newly added parts and the input and output hexagonal star cascaded circuits.

[0050] 1. Topological Deduction of Nine-Switch Converter

[0051] The nine-switch topology is proposed as an alternative to the "back-to-back" 12-switch topology. By combining two three-phase inverters, a three-port converter topology with one DC port and two AC ports is obtained, which can supply two AC loads to operate independently. The two loads can be of the same frequency or different frequencies. Due to the reuse of three switches, there are limitations on the output amplitude and phase shift of the two AC ports. Due to the limitation of the modulation depth, the DC bus voltage increases, which not only puts higher requirements on devices such as switching devices and DC capacitors, but also increases the losses of the converter. However, due to different application fields, the increase in the DC voltage of the nine-switch converter varies greatly. In variable-frequency applications such as electric vehicle drive and dual-motor control, the increase in the DC voltage is relatively large, even up to twice that of the traditional topology scheme. In the same-frequency applications such as uninterruptible power supply and wind power generation, the increase in the DC voltage is relatively small, and this field is suitable for the application of the nine-switch converter. The wind speed of the wind power system is relatively consistent, and the wind power frequency is constant, which can effectively avoid the disadvantage of low utilization rate of the DC side voltage. Therefore, the applicability of the nine-switch converter is relatively high. Thus, a multi-port converter 2 in the form of 3N + 3 is proposed, which can expand the number of AC ports to N. The expansion of its ports requires a significantly reduced number of switches compared to the previous topology. Here, N can be the number m of switch units in the three-phase rectifier arm 21 or the number n of switch units in the three-phase inverter arm 22.

[0052] 2. Medium-voltage and large-capacity upgrade solution

[0053] To meet the requirements of medium-voltage and large-capacity application scenarios, a hexagonal-star converter topology is adopted, which is composed of six switch module back-to-back unit structures connected. Inductors are connected in series between any two sub-modules to suppress the circulating current. This hexagonal-star converter topology adopts synchronous control signals and can achieve good circulating current suppression effect with a relatively small inductor value. In addition, as a symmetric unit structure, under the condition of three-phase balance, the instantaneous power of each sub-module of the hexagonal-star converter is a constant, and no low-frequency power ripple will be generated on the capacitor.

[0054] 3. Back-to-back multi-port converter based on 3N + 3 switch module cascade

[0055] In view of the superiority of the hexagonal star - type cascade, the 3N + 3 multi - port converter 2 is combined with the idea of the hexagonal star - type converter, and a back - to - back multi - port converter based on the cascade of 3N + 3 switch modules is proposed. Similar to the conventional cascade multi - port converter, it is composed of multiple units in cascade. The rectifier front - end of each unit adopts 3m + 3 cascade and is interconnected with the grid side through inductors to achieve bidirectional energy flow. The inverter side of the unit also adopts 3n + 3 cascade. To achieve the expansion of voltage levels and capacity, the topology adopts the connection idea of the hexagonal star. After six inductors are connected in sequence, a hexagon is formed. And because the output is three - phase and the instantaneous power is constant when symmetrical, there is no ripple on the DC side, the system is more stable, and the waveform quality is better.

[0056] Embodiment 2:

[0057] Embodiment 2 is basically the same as Embodiment 1, and the difference lies in:

[0058] See Figure 6 , the AC inputs of the hexagonal star - type cascade circuit 1 at the input end form two groups of three - phase lines and are respectively connected to two wind turbines.

[0059] There are various connection methods for the hexagonal star - type cascade circuit. The following are two examples: For the input or output port x of the 1 - 6 multi - port converters 2, the three - phase lines connected to the 1st multi - port converter 2 are a1 / b1 / c1, the three - phase lines connected to the 2nd multi - port converter 2 are a2 / b2 / c2, and so on.

[0060] Method 1, see Figure 4 , b1 - b2, a2 - a3, c3 - c4, b4 - b5, a5 - a6, c6 - c1. Inductors are connected in series between the two terminals for filtering and eliminating circulating current.

[0061] Method 1, see Figure 5 , c1 - c2, a2 - a3, b3 - b4, c4 - c5, a5 - a6, b6 - b1. Inductors are connected in series between the two terminals for filtering and eliminating circulating current.

Claims

1. A back-to-back multi-port conversion circuit based on cascading of 3N+3 switch modules, characterized in that, Including: m input - end hexagonal star - type cascaded circuits (1), six multi - port converters (2), and n output - end hexagonal star - type cascaded circuits (3). One end of the input - end hexagonal star - type cascaded circuit (1) is connected to a generator through a three - phase line, and the other end of the input - end hexagonal star - type cascaded circuit (1) is respectively connected to the AC input ports of six multi - port converters (2) through six groups of three - phase lines; one end of the output - end hexagonal star - type cascaded circuit (3) is respectively connected to the AC output ports of six multi - port converters (2) through six groups of three - phase lines, and the other end of the output - end hexagonal star - type cascaded circuit (3) outputs through a three - phase line.

2. The back-to-back multi-port conversion circuit based on cascading of 3N + 3 switch modules according to claim 1, wherein The multi - port converter (2) includes a three - phase rectifier bridge arm (21) at the input end, a three - phase inverter bridge arm (22) at the output end, and a transformer T. A capacitor C1 is connected in parallel with the three - phase rectifier bridge arm (21), a capacitor C2 is connected in parallel with the three - phase inverter bridge arm (22). The high - voltage side of the three - phase rectifier bridge arm (21) is connected to the high - voltage side of the three - phase inverter bridge arm (22) after being connected in series with the primary winding of the transformer T, and the low - voltage side of the three - phase rectifier bridge arm (21) is connected to the low - voltage side of the three - phase inverter bridge arm (22) after being connected in series with the secondary winding of the transformer T.

3. A back-to-back multi-port conversion circuit based on cascading of 3N + 3 switch modules according to claim 2, characterized in that, Each phase of the rectifier bridge arm in the three - phase rectifier bridge arm (21) includes: m + 1 switch units connected in series in sequence, and a connection port is arranged between adjacent switch units; the combination of the connection ports of the three - phase rectifier bridge arm forms m three - phase AC input ports. Each phase of the inverter bridge arm in the three - phase inverter bridge arm (22) includes: n + 1 switch units connected in series in sequence, and a connection port is arranged between adjacent switch units; the combination of the connection ports of the three - phase inverter bridge arm forms n three - phase AC output ports.

4. A back - to - back multi - port conversion circuit based on 3N + 3 switch module cascading according to claim 1, characterized in that In the input - end hexagonal star - type cascaded circuit (1), one line is selected from each group of lines as the input to form an AC input with the same frequency and adjacent signals having a phase difference of 60 degrees, and the remaining lines are sequentially connected in series through inductors to form a hexagonal connection to connect six multi - port converters (2).

5. A back-to-back multi-port conversion circuit based on cascading of 3N+3 switch modules according to claim 4, characterized in that, The AC input of the input - end hexagonal star - type cascaded circuit (1) is connected to a wind - power doubly - fed generator.

6. The back-to-back multi-port conversion circuit based on cascading of 3N+3 switch modules according to claim 4, wherein The AC input of the input - end hexagonal star - type cascaded circuit (1) forms two groups of three - phase lines and is respectively connected to two wind turbines.

7. A back - to - back multi - port conversion circuit based on 3N + 3 switch module cascading according to claim 1, characterized in that In the output - end hexagonal star - type cascaded circuit (3), one line is selected from each group of lines as the input to form an AC output with the same frequency and adjacent signals having a phase difference of 60 degrees, and the remaining lines are sequentially connected in series through inductors to form a hexagonal connection to connect six multi - port converters (2); The AC output of the output - end hexagonal star - type cascaded circuit (3) forms two groups of three - phase lines and is connected to the power grid or electrical equipment.

8. An expansion method for a back-to-back multi-port conversion circuit based on cascading of 3N+3 switch modules, characterized in that, The capacity expansion method is based on the 3N + 3 switch module cascaded back - to - back multi - port conversion circuit according to any one of claims 1 to 7, and includes the following steps: S1 Calculate the increment m' of m in the circuit according to the number x of fans installed for capacity expansion. Calculate whether the maximum value of the relevant index parameters of the equipment operation meets the operation safety index of the existing three-phase inverter bridge arm (22) according to the original configuration data of the circuit and the configuration data of the newly added fans. If the safety index is met, only expand the input hexagonal star cascade circuit and the three-phase rectifier bridge arm (21) on the input side to obtain a new circuit design diagram. If the safety index is not met, expand the three-phase inverter bridge arm (22), calculate the minimum value of n that meets the safety index, and then expand the input and output hexagonal star cascade circuits, the three-phase rectifier bridge arm (21), and the three-phase inverter bridge arm (22) simultaneously to obtain a new circuit design diagram. S2 Install the newly added parts and the input and output hexagonal star cascade circuits according to the circuit design.

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